1. The Technical Field
The present invention is directed generally to the field of fiber optics. More particularly, the present invention is directed to a bend-type light injector/extractor for use with multiple optical fibers.
2. The Prior Art
Commercially available single mode-optical fiber couplers typically are made by twisting two uncoated optical fibers together, heating the twisted region, and pulling on the fiber ends to reduce the diameter of the heated and softened region. When the fiber diameters are reduced sufficiently, the fibers"" core modes overlap to produce the desired optical coupling. In principle, this method could be applied to couple to an active fiber without interrupting data transmission through the active fiber, but the procedure would be very risky and would require complex, specialized equipment and precise control over the drawing process. Also, the finished coupler would be permanent and non-adjustable. Further, due to physical constraints, such a fabrication process probably would not be not well suited for use with fiber ribbons having multiple fibers.
Another method for coupling to an active fiber in a non-interrupting manner involves forming a macrobend, typically a constant-radius curve, in a fiber and injecting light through the bend into the core of the fiber. The light injector can consist of a cleaved single mode fiber plus a suitable lens. Injection efficiency (the fraction of the light leaving the injector which actually couples into the core mode of the subject fiber) using the macrobend approach tends to be very low due to inherently poor coupling. In addition, the macrobend techniques usually are applied only to conventional polymer-coated fiber because of the reliability concerns associated with bare fiber. When injecting through the polymer coating, aligning the injector with the single mode core of the bent fiber is extremely difficult, and the resulting coupling is very inefficient.
The basic macrobend technique also can be used to extract light from an optical fiber. For example, some commercial xe2x80x9cfiber identifiersxe2x80x9d used by fiber system installers utilize the macrobend technique for extracting light to identify active fibers and light propagation direction. Some commercially available macrobend-type couplers are available for use in connection with multiple-fiber ribbons.
The macrobend approach typically is not used for permanent couplers because of inherent inefficiencies, plus reliability concerns related to the fiber bending stresses. Bend-type couplers tend to subject the fiber to high bending stresses which are detrimental to the reliability of the fiber because of a phenomenon known as static fatigue. Static fatigue can cause a fiber in bending or tension to fail over time due to the propagation of cracks. The higher the initial stress a fiber is subjected to, the shorter its life is likely to be. Although it is known to use heat to anneal the bent fiber to remove the stress, this technique results in a permanent bend and, therefore, a coupler that is always xe2x80x9conxe2x80x9d and not adjustable. Also, elaborate equipment and precise control are required to practice this technique.
The present invention provides for use in multiple-fiber applications an adjustable fiber optic coupler that allows efficient light injection into one or more active fibers to increase the capacity of a fiber optic system without disrupting existing traffic through the fiber. The present invention thus provides an apparatus for using, for example, multiple single mode injector fibers to simultaneously insert optical signals through the side of and into, for example, the single mode cores of corresponding traffic fibers as an alternative to the conventional approach of severing the traffic fibers to splice in pre-made couplers. For example, the present invention makes possible the insertion of additional wavelengths into multiple fibers (e.g., fiber ribbon) carrying WDM (wavelength division multiplexed) signals, without disrupting the existing traffic. The present invention thereby allows the transmission capacity of active fibers or a network to be upgraded without shutting the underlying system down, and without requiring alternate or xe2x80x9cprotectxe2x80x9d fibers to temporarily carry the traffic over such a network.
The present invention is based on an implementation of what is referred to as the xe2x80x9ctransition effect.xe2x80x9d The transition effect occurs in an optical fiber at a location of curvature discontinuity, such as the point of transition from substantially straight fiber to curved fiber. The transition loss, which occurs for light propagating in either direction, is defined as the amount of light extracted from the fiber (the xe2x80x9clossxe2x80x9d) at the curvature discontinuity, or transition point. However, additional pure bending, or macrobend, losses of the signal on the affected traffic fiber (manifested in discrete rays of light) will occur if the curve is tight enough and/or long enough. The overall insertion loss includes the sum of the transition loss and the pure bending losses.
A light injector according to a preferred embodiment of the present invention includes a plurality of traffic fibers, a corresponding plurality of injector fibers, a substrate which holds corresponding traffic and injector fibers in proper alignment with each other, a clamp which secures the traffic fibers to the substrate, and a depressor which imparts a bend to a portion of each of the traffic fibers. Each injector fiber preferably includes a lens for directing light to be injected to the traffic fiber, this lens being connected to the fiber via an intervening coreless fiber spacer. Preferably, the depressor is adjustable. That is, it can be moved towards or away from the clamp so as to change the magnitude of the bend it imparts to the traffic fibers.
A multi-fiber injector according to the present invention is adjustable, highly-efficient, non-invasive, imposes small insertion loss, and can handle high data rates, as well as WDM signals. In addition, such a device is reciprocal in nature. That is, it can be used in reverse as a light extractor to extract, or drop, an adjustable fraction of light from multiple traffic fiber cores into corresponding detectors or other single mode fibers.